The determination of new forms of base modification in nucleic acids is an essential step in understanding structure-function relationships. New nucleosides will be isolated and their structures established, from transfer RNA of archaebacteria, E. coli, rat and beef liver, mouse L-M cells, and Chlorella pyrenoidosa. Structural studies will be carried out on modified bases from the bacteriophage RL38J1. The primary methods employed will be high performance liquid chromatography and mass spectrometry, both separately and directly combined.